The invention relates to the preparation of polypropylene. In particular, the invention relates to the preparation of polypropylene that has isotactic and atactic stereoblock sequences.
Polypropylene differs from polyethylene because there is a methyl group attached to every other carbon atom of the polypropylene backbone. Depending upon the locations of the methyl groups, polypropylene can be divided into three types: isotactic, syndiotactic, and atactic.
Isotactic polypropylene has been commercially produced for decades with Ziegler catalysts. Single-site catalysts are also suitable for the preparation of isotactic polypropylene. For instance, WO 99/24446 teaches the use of bridged indenoindolyl-based single-site catalysts to prepare polypropylene that has an isotactic content greater than 90%. Isotactic polypropylene readily forms crystalline structure. It has excellent chemical and heat resistance and has been mainly used for textile fibers and films.
Unlike isotactic polypropylene, atactic polypropylene is amorphous. It has better toughness but less chemical and heat resistance than isotactic polypropylene. It is mainly used in adhesives. Although atactic polypropylene can be made directly by polymerization (see, e.g., U.S. Pat. No. 5,945,496), it is usually a by-product of isotactic polypropylene production.
It is of significant interest to combine isotactic and atactic polypropylene because they have complementary properties. However, physically blending these polymers offers little benefit because they are not readily compatible. Preparation of polypropylene having both isotactic and atactic sequences in the same polymer chain would be an ideal way to combine these two polymers. However, it is difficult to find a catalyst that can alternately grow isotactic and atactic sequences.
U.S. Pat. No. 5,594,080 teaches the use of aryl-indenyl-based single-site catalysts to prepare polypropylene that has both isotactic and atactic sequences. The polypropylene contains about 20% or less of isotactic structure.
U.S. Pat. No. 5,747,621 also teaches the preparation of polypropylene that has both isotactic and atactic components. However, these components are mainly not incorporated into the same polymer chain. Rather, they are physically blended.
U.S. Pat. No. 5,756,614 teaches the preparation of stereoblock polypropylene using an asymmetric stereorigid metallocene catalyst. The catalyst possesses two exchangeable catalytic sites with different stereochemical reactivity. Thus, an isotactic/atactic stereoblock polypropylene is prepared. The polymer shows promising performance as a thermoplastic elastomeric material. However, the catalyst is complicated and expensive to make.
In sum, there is an increasing interest in ways to make isotactic-atactic stereoblock polypropylene. Ideally, the preparation would use a readily available and inexpensive single-site catalyst.
The invention is a propylene polymerization process. The process uses a Group 3-5 transition metal catalyst that has two non-bridged indenoindolyl ligands. The catalyst is activated. The invention provides a simple but effective route to polypropylene that has isotactic and atactic stereoblock sequences. In contrast to known processes, the process of the invention does not require complicated bridged catalysts; nor does it require blending an isotactic polypropylene with atactic polypropylene. The polypropylene produced has an isotactic pentad (mmmm) content within the range of about 10 mole % to about 70 mole % and is suitable for the use as thermoplastic elastomeric material.
The process of the invention comprises polymerizing propylene in the presence of a Group 3-5 transition metal catalyst. The catalyst has two indenoindolyl ligands. By xe2x80x9cindenoindole compound,xe2x80x9d we mean an organic compound that has both indole and indene rings. The five-membered rings from each are fused, i.e., they share two carbon atoms. The indenoindolyl ligands are not bridged to each other.
The catalyst preferably has the general structure 
where M is a Group 3-5 transition metal. Preferably, M is zirconium. The indenoindolyl ligands, L1 and L2, are Π-bonded to M. L1 and L2 can be the same or different and have the following alternative structures: 
R1 is preferably selected from the group consisting of alkyl, aryl, aralkyl, and silyl groups. Examples are methyl, t-butyl, phenyl, and tri-methyl silyl groups. R2 through R10 are the same or different and are preferably selected from the group consisting of hydrogen, alkyl, aryl, aralkyl, silyl, halogen, alkoxy, aryloxy, siloxy, thioether, nitro, amino groups, and the like.
The catalyst has two other ligands, X1 and X2. X1 and X2 can be the same or different. They are preferably selected from the group consisting of halogen, alkoxy, aryloxy, siloxy, dialkylamino, diarylamino, and hydrocarbyl groups. Labile ligands such as halogen are particularly preferred.
Examples of suitable catalysts include bis-(2-chloro-5-phenyl-5,10-dihydroindeno [1,2-b]indolyl)zirconium dichloride (Structure I), bis-(5-phenyl-5,10 -dihydroindeno[1,2-b]indolyl)zirconium dichloride (Structure II), bis-(5,8-dimethyl-5,10-dihydroindeno[1,2-b]indolyl)zirconium dichloride (Structure III), and bis-(5-trimethylsilyl-8-methyl-5,10-dihydroindeno[1,2-b]indolyl)zirconium dichloride (Structure IV). 
The catalysts can be prepared by any known method. For instance, co-pending application Ser. No. 09/417,510, now U.S. Pat. No. 6,232,260, the teachings of which are incorporated herein by reference, teaches in great detail how to prepare indenoindole-based catalysts. For instance, Catalyst III can be made according to the following scheme: 
The catalysts are activated. Suitable activators include alumoxanes, alkyl aluminums, alkyl aluminum halides, anionic compounds of boron or aluminum, trialkylboron and triarylboron compounds. Examples include methyl alumoxane (MAO), polymeric MAO (PMAO), ethyl alumoxane, diisobutyl alumoxane, triethylaluminum, diethyl aluminum chloride, trimethylaluminum, triisobutyl aluminum, lithiumtetrakis(pentafluorophenyl) borate, lithium tetrakis(pentafluoro-phenyl)aluminate, dimethylanilinium tetrakis (pentafluorophenyl)borate, trityl tetrakis (pentafluorophenyl)borate, tris(pentafluorophenyl)borane, triphenylborane, tri-n-octylborane, the like, and mixtures thereof.
Selection of activator depends on many factors including the catalyst used and the desired isotactic/atactic ratio of the polypropylene product. For instance, when bis(2-chloro-5-phenyl-5,10-dihydroindeno[1,2-b]indolyl) zirconium dichloride is used as a catalyst and MAO as an activator, the polypropylene produced has an isotactic pentad content of about 50% (see Example 1), while using a combination of triisobutyl aluminum and trityltetrakispentafluorophenylborate as activator, the isotactic pentad content is about 20% (Example 12).
Optionally, the catalyst is immobilized on a support. The support is preferably a porous material such as inorganic oxides and chlorides, and organic polymer resins. Preferred inorganic oxides include oxides of Group 2, 3, 4, 5, 13, or 14 elements. Preferred supports include silica, alumina, silica-aluminas, magnesias, titania, zirconia, magnesium chloride, and crosslinked polystyrene.
Many types of propylene polymerization processes can be used. The process can be practiced in the gas phase, bulk, solution, or slurry. The polymerization can be performed over a wide temperature range. Preferably, the temperature is within the range of about 0xc2x0 C. to about 150xc2x0 C. A more preferred range is from about 25xc2x0 C. to about 100xc2x0 C.
The process of the invention gives polypropylene products having controlled stereochemical configurations and physical properties. It provides a simple but effective way to tailor the isotactic/atactic ratio of polypropylene. The invention eliminates the need of complicated bridged catalysts. The polypropylene of the process has a broad range of isotactic pentad content (about 10 mole % to about 70 mole %) and are useful materials for films, adhesives, and other elastomeric objects.
The following examples merely illustrate the invention. Those skilled in the art will recognize many variations that are within the spirit of the invention and scope of the claims.